In our latest post, authors Tom Martin and Penn Lloyd explore what makes songbirds different in their breeding cycles. Tom, a retired scientist of the University of Montana, and Penn, an ecological consultant in Queensland Australia, share insight behind their paper: “Potential drivers of differences in breeding phenology as a component of life history strategies among coexisting species”. They help explain the causes and consequences of breeding variation in songbirds and demonstrate how a passion for birds can lead to new revelations!
About the paper
Timing of breeding can have a large impact on reproductive success and species survival.. It is normal to think that changes in food supply, linked to seasonal changes in climate, determine the timing of breeding. Yet, bird species that live in the same habitat with the same vegetation, climate, and predators still differ in when they start and stop breeding. To investigate why, we studied a community of 16 songbird species that coexist in a South African semi-arid coastal shrubland. Despite nesting close together, these birds differed by nearly 1.5 months in the dates of when they started or stopped breeding. As generally seen among birds, most nesting attempts failed due to high nest predation. In this community, predation risk generally increased from late winter into summer. Snakes were important nest predators, and their activity increases with warming temperatures. We found that species subjected to increases in nest predation rates across the season and greater average nest predation rates, started and stopped breeding earlier than other species, thereby reducing nest predation risks.
We also found that adult mortality influenced the timing of breeding, with longer-lived species starting (breeding) earlier. Longer- lived birds have fewer young that require less food soby breeding earlier when food is less abundant, they may avoid the high nest predation later in the season. For example, the Cape Robin-Chat (Dessonornis caffra) had the lowest adult mortality rate. These birds havea typically small brood size of 2 young and started breeding much earlier than any other species. On the other hand, we noted that shorter-lived species had up to 5 young per nest. A larger brood size will require more food, so those species bred later (despite greater nest predation risk), when food was more abundant. The evolved timing of breeding in this community reflects a delicate balance between seasonal changes in nest mortality risk and food availability. We concluded that this cost-benefit juggling act is, in turn, influenced by longevity and brood size. Our results highlight those evolved differences in timing of breeding among coexisting species is an important component of life history strategies.
Individuals such as this Bokmakierie (Telophorus zeylonus) were colour-banded to monitor adult survival. (Credit: Penn Lloyd)
The Cape Robin-chat (Dessonornis caffra), seen here, was the longest-lived of the species studied, with a typical brood size of two young. (Credit: Tom Martin)
About the research
The study site was uniquely suited to examining the life history traits of coexisting species. This study has been my (Tom Martin) long-standing research focus and incorporated the impressive talents of Penn Lloyd, a local scientist to the area. The breeding bird community was surprisingly diverse and abundant for a semi-arid environment, and the breeding season ranged from cold and wet conditions in late winter to hot and dry conditions in early summer. The vegetation wasn’t very high, which allowed easy access to the nests of all species in the community and facilitated the capture, banding and resighting of birds for monitoring adult survival. We considered this a great advantage since there are not too many environments where a field team of 5-6 can monitor up to 1,500 nests over a 3-month period! Situated in a protected nature reserve, the site also supported a diverse community of bird nest predators, including 3 birds (a crow, shrike and harrier), 3 mammals (2 native cats and a mongoose) and at least 5 species of snake, including one that eats only bird eggs (which we managed to get a picture of!) and another specializing on bird nestlings and adults.
The low coastal shrubland gave easy access to the nests of all species in the community. (Credit: Penn Lloyd)
A common egg-eating snake (Dasypeltis scabra) found in a Bar-throated Apalis nest just after swallowing the egg, which can be seen by the bulge in its throat! (Credit: Tom Martin)
About the authors
I (Tom Martin) am a retired scientist who worked for the USGS Cooperative Research Units and the University of Montana. I now travel to see birds in their field environments throughout the world, while also periodically writing up diverse data. Penn Lloyd works as an ecological consultant in Queensland, Australia but manages to sustain his passion for studying the breeding ecology of birds in his spare time. We have learned that the skills developed in research, particularly problem solving, writing and continually questioning the natural world, help in our deeper understanding of how ecological systems respond to the environment. It can be rewarding and transfers well to commercial consulting!
Like the blog post? Read the research here.